This research program is designed: 1) to study the development of the sensory epithelia of the inner ear and the cytodifferentiation of the sensory cells at the three phases of embryonic development, and 2) to determine the factors influencing the histogenesis and cytodifferentiation of the sensory epithelia. The embryonic chick inner ear is being used as a model for these studies which begin with the 4-day-old otocyst. Organ cultures, in vivo specimens, histochemical procedures, differential staining and electron microscopy are being used in these studies. 1) Phase 1. This is the proliferative phase in which presumptive sensory epithelia become localized. Phase 2. This is the mid-point in development. Morphological formations of the crista and basilar papilla are established. A few completely differentiated hair cells with typical hair structures can be demonstrated with the electron microscope. Phase 3. Completely formed and differentiated cristae and basilar papillae can be observed in both organ cultures and in vivo specimens. The development of sensory epithelia in organ culture closely parallels that in vivo. 2) Experiments are designed to determine a) the requirement for mesencyme for histogenesis of the sensory epithelia, and b) the requirement for innervation for cytodifferentiation of the sensory cells. An additional study is proposed in which the development of the otocyst and the acoustic ganglion, beginning with the placodal stage, will be followed.